The subject matter disclosed herein relates generally to a method and system for online diagnostic evaluation of a motor driven mechanical system and, more specifically, to a method and system for monitoring a frequency spectrum obtained from a measured or commanded signal corresponding to operation of a motor in the mechanical system to identify a probability and severity of one of several fault conditions present in the motor.
Electrical rotating machines, such as electric motors or generators, have become widespread and are found in numerous applications and configurations. Electric machines include a stationary component (i.e., the stator) and a rotating component (i.e., the rotor). In electric motors, a magnetic field is established in the rotor, for example via magnets mounted to the rotor or via an electrical current applied to or induced in a coil wound on the rotor. A second, rotating magnetic field is established as a result of the application of a controlled voltage to the stator, and the rotation of the magnetic field in the stator causes the magnetic field in the rotor to rotate, thereby causing rotation of the rotor. A shaft or other drive member is mounted to the rotor and extends outside the rotor housing providing a mechanical coupling to a device, such as a gearbox, pump, or fan that is to be driven as the rotor rotates.
As is known to those skilled in the art, motor drives are utilized to control operation of a motor. Motor drives may be provided to convert input power, from either an alternating current (AC) source or a direct current (DC) source, to the controlled voltage applied to the stator. In certain applications, high performance of the motor and the controlled machine is desired. For example, a servo motor may position a machine tool with high speed and tight position tolerances for repeatable manufacturing of components. The servo motors may be mounted to a machine and coupled, via a gearbox to a drive member, such as a ball-screw or a rack and pinion used to position one axis of the machine.
Due to the rotational nature of an electric machine, fault conditions may present themselves as vibrations or resonances within the mechanical system. The fault conditions may be a result of normal wear, such as bearing wear, or due to application specific conditions, such as a misalignment between the motor shaft and the gearbox, a compliant mechanical load, or torque ripple resulting from the motor construction. These conditions may generate an imbalance on the rotor shaft and can result in vibrations or resonance being generated within the control system. Vibration in a rotational mechanical system may accelerate wear and increase maintenance frequency and expense.
Thus, it would be desirable to provide a method and system to monitor and identify vibrations in a rotational mechanical system to provide an early indication of a problem in the system and to extend machine life.
Historically, vibration detection has been performed external to the motor drive, for example, by data acquisition and analysis systems. A vibration sensor may be mounted to the motor and a signal corresponding to vibration is generated and transmitted to the data acquisition and analysis system. Vibration sensors, however, introduce additional expense and configuration requirements. A vibration sensor requires additional clearance for installation and must be aligned such that the transducers within the sensor detect vibration in a desired direction, such as axially or radially with respect to the motor.
As an alternative to a vibration sensor, the motor drive may be configured to transmit data to the data acquisition and analysis system, where the data corresponds to operation of the motor or motor drive. When vibration occurs, the current feedback signal, for example, may include harmonic content at the frequency of vibration. By transmitting such data to the data acquisition and analysis system, the system may analyze the content of the signal and provide a frequency response of the signal which identifies the harmonic content. However, complex control systems, such as a machine tool or a process line include multiple controlled motors or axes of motion. As the number of controlled axes increase, the volume of data being transmitted over a data bus to the data acquisition and analysis system by multiple motor drives similarly increases. In order to detect vibration at a particular frequency, the data must be sampled at a rate at least twice that of the frequency to be detected and preferably the data is sampled at a rate that is an order of magnitude greater than the frequency to be detected. To detect a vibration, for example, at 500 Hz, the data must be sampled at a minimum of one thousand times per second and preferably at five thousand times per second. Each sample for each motor drive must then be transmitted to the data acquisition and analysis system using a significant amount of communications bandwidth both within the motor drive and on an industrial network provided between the data acquisition system and the motor drives.
Thus, it would be desirable to provide an improved method and system for analyzing motor performance to detect vibration of an electric machine controlled by a motor drive.
As indicated above, a number of different conditions, such as bearing wear, misalignment, or a bent shaft, may cause vibration in the machine. However, many of the different conditions that cause vibrations exhibit different vibration characteristics.
For example, the vibration may vary with load or vary as a function of the speed of the motor. The different conditions may generate a vibration that has a frequency equal to or proportional to the speed of the motor or that has a frequency that is equal to or proportional to multiples of the speed of the motor.
Thus, in addition to detecting vibration, it would be desirable to provide an improved method and system for identifying various fault conditions in a rotating machine operating at variable speeds in a mechanical system as a function of the characteristics of the detected vibration.